People should realise that Matt also has for profit company Automatic and it's mostly defending his cash cow rather than spirit of open source.

Also speaking about originaly MIT licensed editor code which suddenly becomes GPL: Taking someones work adding modifications on top and slapping different license on top wildly unethical. As it benefits only the person who slapped GPL on top. As I know if you use GPL licensed code you must have GPL license. At least in wordpress case: https://wordpress.org/about/license/. So author can not use the improvements Automatic engineers made while using his library for profit.